1. Field of the Invention
The present invention relates to a photocathode applicable to an image-pickup tube or photometry device and a phototube having the same and, more particularly, a photocathode having a plurality of layers each mainly containing a compound semiconductor material and functioning so as to emit photoelectrons which are excited by incident light.
2. Related Background Art
Conventionally, a compound semiconductor layer of CsTe is applied to a photocathode for the UV range. The spectral sensitivity characteristic of this photocathode has a high radiation sensitivity to incident light having a wavelength of about 180 to 320 nm. However, to improve the quantum efficiency or absorption edge characteristic in the long wavelength side within the wavelength range of incident light, there is a decisive difficulty caused by a defect called a color center.
As a photocathode for the UV range for which various operation characteristics are expected to be improved as compared with the photocathode having the CsTe layer, a photocathode having a compound semiconductor layer mainly containing Ga.sub.1-x Al.sub.x N (0&lt;x&lt;1) is available. The spectral sensitivity characteristic of this photocathode is adjusted by changing the absorption edge characteristic in the long wavelength side within the incident wavelength range of about 200 to 350 nm in correspondence with the composition of Ga.sub.1-x Al.sub.x N, i.e., an alloy of AlN and GaN.
Prior arts associated with such a photocathode having a layer mainly containing a compound semiconductor material of Ga.sub.1-x Al.sub.x N are disclosed in, e.g.,
"U.S. Pat. No. 3,387,161 (1968)", PA1 "U.S. Pat. No. 3,986,065 (1976)", and PA1 Japanese Patent Laid-Open No. 61-267374.